The present invention relates generally to managing firing sequence phase transitions during skip fire and other cylinder output level modulation operation of an engine. The invention is also useful in applications where it is desirable to transition from dynamic skip fire engine control into fixed cylinder-based firing patterns.
Skip fire engine control is understood to offer a number of benefits including the potential of increased fuel efficiency. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, for example, a particular cylinder may be fired during one firing opportunity and then may be skipped during the next firing opportunity and then selectively skipped or fired during the next. Skip fire engine operation is distinguished from conventional variable displacement engine control in which a designated set of cylinders are deactivated substantially simultaneously during certain low-load operating conditions and remain deactivated as long as the engine remains in the same displacement mode. Thus, the sequence of specific cylinders firings will always be exactly the same for each engine cycle during operation in any particular variable displacement mode (so long as the engine maintains the same displacement), whereas that is often not the case during skip fire operation. For example, an 8-cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it operates using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three fixed displacement modes.
In general, skip fire engine operation facilitates finer control of the effective engine displacement than is possible using a conventional variable displacement approach. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓th of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. Conceptually, virtually any effective displacement can be obtained using skip fire control, although in practice most implementations restrict operation to a set of available firing fractions, sequences or patterns. The Applicant, Tula Technology, Inc., has filed a number of patents describing various approaches to skip fire control. By way of example, U.S. Pat. Nos. 8,099,224; 8,464,690; 8,651,091; 8,839,766; 8,869,773; 9,020,735; 9,086,020; 9,120,478; 9,175,613; 9,200,575; 9,200,587; 9,291,106; 9,399,964, and others describe a variety of engine controllers that make it practical to operate a wide variety of internal combustion engines in a dynamic skip fire operational mode. Each of these patents and patent applications is incorporated herein by reference.
In some applications referred to as multi-level skip fire, individual working cycles that are fired may be purposely operated at different cylinder outputs levels—that is, using purposefully different air charge and corresponding fueling levels. By way of example, U.S. Pat. No. 9,399,964 (which is incorporated herein by reference) describes some such approaches. The individual cylinder control concepts used in dynamic skip fire can also be applied to dynamic multi-charge level engine operation in which all cylinders are fired, but individual working cycles are purposely operated at different cylinder output levels. Dynamic skip fire and dynamic multi-charge level engine operation may collectively be considered different types of cylinder output level modulation engine operation in which the output of each working cycle (e.g., skip/fire, high/low, skip/high/low, etc.) is dynamically determined during operation of the engine, typically on an individual cylinder working cycle by working cycle (firing opportunity by firing opportunity) basis. It should be appreciated that cylinder output level engine operation is different than conventional variable displacement in which when the engine enters a reduced displacement operational state, a defined set of cylinders are operated in generally the same manner until the engine transitions to a different operational state.
Some firing fractions used while operating in a dynamic skip fire mode will result in the same cylinders being fired each engine cycle. When this occurs, it may sometimes be desirable to control which specific cylinders are being fired. Similarly during multi-level skip fire or multi-charge level operation of an engine, certain effective firing fractions may cause one or more specific cylinders to always be fired high or to always be fired low. Again, in such circumstances it may sometimes be desirable to be able to specify the specific cylinder(s) that are consistently fired in the same state.
The present application describes techniques that can be used to manage the phase of a firing sequence and is particularly useful in conjunction with dynamic skip fire control.